US7599449B2 - Hybrid modulus blind equalization for quadrature amplitude modulation (QAM) receivers - Google Patents
Hybrid modulus blind equalization for quadrature amplitude modulation (QAM) receivers Download PDFInfo
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- 238000004891 communication Methods 0.000 description 4
- 239000000969 carrier Substances 0.000 description 3
- 238000005516 engineering process Methods 0.000 description 3
- 238000013459 approach Methods 0.000 description 2
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
- H04L25/03006—Arrangements for removing intersymbol interference
- H04L25/03012—Arrangements for removing intersymbol interference operating in the time domain
- H04L25/03019—Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception
- H04L25/03038—Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception with a non-recursive structure
- H04L25/0305—Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception with a non-recursive structure using blind adaptation
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
- H04L25/03006—Arrangements for removing intersymbol interference
- H04L2025/0335—Arrangements for removing intersymbol interference characterised by the type of transmission
- H04L2025/03375—Passband transmission
- H04L2025/0342—QAM
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
- H04L25/03006—Arrangements for removing intersymbol interference
- H04L2025/03592—Adaptation methods
- H04L2025/03598—Algorithms
- H04L2025/03611—Iterative algorithms
- H04L2025/03617—Time recursive algorithms
- H04L2025/0363—Feature restoration, e.g. constant modulus
Definitions
- At least some embodiments of the invention relate to blind equalizer in general and, particularly but not exclusively to blind equalizer for Quadrature Amplitude Modulation (QAM) receivers.
- QAM Quadrature Amplitude Modulation
- Quadrature Amplitude Modulation can be used to represent data by changing, or modulating, the amplitude of two carrier waves, which are out of phase with each other by 90 degrees and are thus called quadrature carriers.
- the quadrature carriers can be modulated in amplitude to represent digital symbols being transmitted.
- the amplitude of modulation in the two quadrature carriers for a symbol is represented along the real and imaginary axes in a complex plane, the symbol can be represented as a point in the complex plane.
- a set of symbols used in a QAM scheme can be collective called a constellation.
- a constellation diagram shows the set of symbols in the complex plane.
- a rectangular QAM constellation includes a set of symbols arranged on a rectangular grid. Rectangular QAM constellations may not be optimal in that the points in the constellation do not maximally space from each other. None rectangular QAM constellations may also be used to improve separation, but they are harder to modulate and demodulate than rectangular QAM constellations.
- Signals transmitted through a transmission channel suffer from non-ideal channel characteristics such as Additive White Gaussian Noise (AWGN), Inter Symbol Interference (ISI), fading, and phase distortion, etc.
- AWGN Additive White Gaussian Noise
- ISI Inter Symbol Interference
- fading fade
- phase distortion etc.
- the transmitted signals can be distorted by the channel characteristics, which is typically unknown.
- Equalization is a technique used to reduce distortion and compensate for signal loss (attenuation).
- an equalizer uses an adjustable filter which is adjusted to compensate the unknown channel characteristics.
- Blind equalization is a type of technology, which does not use any training sequence and thus reduces the system overhead. Blind equalization has been widely used to adapt the receiver to the channel conditions. Many blind equalization algorithms have been developed.
- CMA Constant Modulus Algorithm
- Constant Modulus Algorithm is a simple and effective way to achieve channel equalization.
- a Constant Modulus Algorithm minimizes an error function for equalization.
- the error function is based on the difference between the equalizer output and a constant constellation radius: [
- y is the equalizer output
- K a constant
- p and q are typically integers.
- FIG. 1 shows a block diagram of a conventional CMA-based blind equalizer.
- the adjustable filter ( 101 ) has a number of coefficients, also referred as tap weights, which determine the transfer function of the equalizer.
- the input signal to the adjustable filter ( 101 ) may be distorted due to the unknown channel characteristics.
- the adaptation engine ( 109 ) adjusts the tap weights according to the error generator ( 105 ) to reduce the error between the output of the adjustable filter ( 101 ) and the constant modulus ( 107 ).
- the decision engine ( 103 ) identifies the symbol being transmitted from the output of the adjustable filter ( 101 ) to generate the decision output ( 103 ).
- the tap weights are continuously adjusted by the adaptation engine ( 109 ) to reduce the error until the equalizer converges.
- a CMA equalizer has a large convergence range. However, since a CMA equalizer uses only one modulus, a large amount of residual mean square error (MSE) may exist after convergence, due to adaptation noise. The residual error may cause decision errors for high order QAM signals.
- MSE mean square error
- the conventional CMA was modified to develop improved algorithms, such as a Sato algorithm (see, e.g., M. Goursat, et al., in “Blind Equalizers, IEEE Trans. of Communications, Vol. COM-28, August 1984) and a “stop-and-go” decision-directed algorithm (see, e.g., G. Picchi, et al, in “Blind equalization and carrier recovery using a ‘stop-and-go’ decision-directed algorithm, IEEE Trans. Of Communications, Vol. COM-35, in September 1987).
- a Sato algorithm see, e.g., M. Goursat, et al., in “Blind Equalizers, IEEE Trans. of Communications, Vol. COM-28, August 1984
- a stop-and-go decision-directed algorithm see, e.g., G. Picchi, et al, in “Blind equalization and carrier recovery using a ‘stop-and-go’ decision-directed algorithm, IEEE Trans. Of Communications, Vol. COM-35
- M. J. Ready and R. P. Gooch describes a multi-modulus algorithm in “blind equalization based on radius directed adaptation, Proc. 1990 IEEE Int. Conf. Acoust., Speech, Signal Processing, Albuquerque, N.Mex., PP 1699-1702, 1990, in which radius directed adaptation is based on the known modulus of the constellation symbol radii.
- the error function is based on the difference between the equalizer output and the nearest constellation radius: [
- y is the equalizer output
- K d is the radii of the nearest constellation symbol for the equalizer output y
- common values for (p, q) are (1, 1), (1, 2), (2, 1), (2, 2), etc.
- One embodiment of the present invention includes a Quadrature Amplitude Modulation (QAM) signal receiver that includes a filter to reduce error in equalization, the filter to output a QAM signal; a decision engine coupled to the filter to determine a symbol based on the QAM signal; a first error generator coupled to the filter to compute a first error signal based on the QAM signal and a constant; a second error generator coupled to the filter and the decision engine to compute a second error signal based on the QAM signal and the determined symbol; an error combinator coupled to the first and second error generators to generate a combined error signal from the first and second error signals; and an adaptation engine coupled with the error combinator and the filter to reduce a equalization error according to the combined error signal.
- QAM Quadrature Amplitude Modulation
- the first error generator includes a constant modulus algorithm (CMA) error generator.
- CMA constant modulus algorithm
- the second error generator includes a decision modulus algorithm (DMA) error generator.
- DMA decision modulus algorithm
- the error combinator combines the first and second error signals according to a difference between the QAM signal and the determined symbol.
- the error combinator applies a first weight on the first error signal and a second weight on the second error signal to generate the combined error signal; and the first and second weights are determined based on the difference between the QAM signal and the determined symbol.
- the first weight decreases relative to the second weight to zero as the difference between the QAM signal and the determined symbol decreases; and the second weight decreases relative to the first weight to zero as the difference between the QAM signal and the determined symbol increases to above a threshold.
- One embodiment of the invention includes a method that includes receiving a Quadrature Amplitude Modulation (QAM) signal (e.g., in a decision engine); determining a symbol corresponding to the received QAM signal (e.g., received in the decision engine); computing a first error in equalization based on a constant modulus and a second error in equalization based on the determined symbol; and adjusting a filter to reduce error in equalization according to the first error and the second error.
- QAM Quadrature Amplitude Modulation
- the first error is based on a constant modulus algorithm (CMA) error.
- CMA constant modulus algorithm
- the second error is based on a difference between the modulus of the determined symbol and the modulus of the received QAM signal.
- the step of adjusting the filter includes combining the first error and the second error to adjust the filter.
- the step of combining the first error and the second error includes weighting the first error against the second error according to a difference between the determined symbol and the received QAM signal.
- a weight for the first error decreases relative to a weight for the second error when the difference between the determined symbol and the received QAM signal decreases.
- the said combining the first error and the second error includes determining a weighted average of the first error and the second error according to a difference between the determined symbol and the received QAM signal.
- a weight for the first error decreases to zero when the difference between the determined symbol and the received QAM signal decreases to below a threshold.
- the weight for the second error decreases to zero when the difference between the determined symbol and the received QAM signal increases to above a threshold.
- One embodiment of the invention includes a circuit including means for receiving a Quadrature Amplitude Modulation (QAM) signal; means for determining a symbol corresponding to the received QAM signal; means for computing a first error in equalization based on a constant modulus and a second error in equalization based on the determined symbol; and means for adjusting a filter to reduce error in equalization according to the first error and the second error.
- QAM Quadrature Amplitude Modulation
- the first error is based on a constant modulus algorithm (CMA) error
- the second error is based on a difference between the modulus of the determined symbol and the modulus of the received QAM signal.
- CMA constant modulus algorithm
- the means for adjusting the filter includes means for weighting the first error against the second error according to an confidence level in the determined symbol.
- a weight for the first error decreases relative to a weight for the second error when the confidence level increases.
- the weight for the first error decreases to zero when the confidence level increases to above a first threshold; and the weight for the second error decreases to zero when the confidence level decreases to below a second threshold.
- FIG. 1 shows a block diagram of a conventional receiver with a Constant Modulus Algorithm (CMA) based blind equalizer.
- CMA Constant Modulus Algorithm
- FIG. 2 shows a block diagram of a Quadrature Amplitude Modulation (QAM) receiver according to one embodiment of the invention.
- QAM Quadrature Amplitude Modulation
- FIGS. 3-4 shows example block diagrams of blind equalizers according to embodiments of the invention.
- FIG. 5 shows example weight functions according to an embodiment of the invention.
- FIG. 6 shows a flow diagram of a process in a blind equalizer according to one embodiment of the invention.
- One embodiment of the present invention provides a hybrid modulus blind equalization algorithm with small residual mean square error and large convergence range (e.g., for high order QAM constellations).
- DMA Decision Modulus Algorithm
- a DMA error generator may compute the error using the modulus of the input of the QAM decision engine and the modulus of the output of the QAM decision engine.
- a DMA may achieve zero residual error.
- the convergence range of the DMA can be fairly limited, especially when the QAM size increases.
- One embodiment of the invention provides a hybrid scheme which has the advantage of a large convergence range and the advantage of zero residual error upon convergence.
- One embodiment of the invention includes a hybrid modulus algorithm for blind equalization, which uses an adaptation error signal generated by a combination of a CMA (Constant Modulus Algorithm) error and a DMA (Decision Modulus Algorithm) error.
- the hybrid modulus algorithm has a reduced residual mean square error (MSE) after convergence, while having the same acquisition ability as a CMA-based blind equalizer.
- the hybrid modulus algorithm can be used for digital QAM (Quadrature Amplitude Modulation) signal constellations, especially for high constellation sizes.
- a decision modulus algorithm is used together with a constant modulus algorithm (CMA); the adaptation error signal is calculated based on both the DMA error and the CMA error; and a weight controller is used to automatically adjust the ratio of the two errors according to the confidence level.
- the adaptive weight controller determines how the hybrid error signal is composed by the CMA error and the DMA error.
- FIG. 2 shows a block diagram of a Quadrature Amplitude Modulation (QAM) receiver according to one embodiment of the invention.
- QAM Quadrature Amplitude Modulation
- the input signal to the equalizer is corrected using the adjustable filter ( 201 ).
- the output of the adjustable filter ( 201 ) is used by the decision engine ( 203 ) as an input QAM signal to determine the decision output.
- one error generator ( 205 ) is based on a constant modulus ( 211 ); and another error generator ( 207 ) is based on the decision output.
- the error signals from the error generators ( 205 and 207 ) are combined to drive the adaptation engine ( 209 ), which adjusts the adjustable filter ( 201 ) to reduce equalization error.
- the constant modulus based error generator ( 205 ) can be designed according to a CMA, which compares the output of the adjustable filter with a pre-calculated constant modulus to generate an error signal. If the error signal from the error generator ( 205 ) were used to drive the adaptation engine ( 209 ) alone, the equalizer would have a large convergence range and a large residual MSE after convergence.
- the decision based error generator ( 207 ) compares the output of the adjustable filter with a decision output to generate an error signal. If the error signal from the error generator ( 207 ) were used to drive the adaptation engine ( 209 ) alone, the equalizer would have a small convergence range and a small residual MSE after convergence.
- an error combinator ( 213 ) is used to combine the error signals from both the error generators ( 205 and 207 ) to drive the adaptation engine ( 209 ).
- the error signal generated from the decision based error generator ( 207 ) is used for small residual MSE after convergence; and the error signal generated from the constant modulus based error generator ( 205 ) is used for large convergence range.
- the error combinator ( 213 ) mixes the error signals for the error generators ( 205 and 207 ) according to the confidence level in the decision output of the decision engine.
- the confidence level may be determined based on the difference between the input and output of the decision engine ( 203 ).
- the confidence level increase, more error signals from the decision based error generator ( 207 ) is used to drive the adaptation engine ( 209 ) than the error signals from the constant modulus based error generator ( 211 ).
- the confidence level decreases more error signals from the constant modulus based error generator ( 211 ) is used to drive the adaptation engine ( 209 ) than the error signals from the decision based error generator ( 207 ).
- the proposed scheme uses an adaptation error signal generated by a mixture of the CMA error and the DMA error.
- a weight controller automatically determines the weights of the two errors according to their respective confidence levels.
- FIGS. 3-4 shows example block diagrams of blind equalizers according to embodiments of the invention.
- the error signals from the error generators ( 305 and 307 ) are weighted using adjustable scalers ( 313 and 315 ) and summed using an adder ( 319 ).
- the output of the adder ( 319 ) drives the adaptation engine ( 309 ) to adjust the filter ( 301 ) to reduce the equalization error.
- the weight controller ( 317 ) determines weights for error signals from the error generator ( 305 and 307 ) according to a confidence level indicator computed based on the input to and output from the decision engine ( 303 ).
- the confidence level is high; the weight for the error determined based on the decision output is higher than the weight for the error determined based on the constant modulus ( 311 ).
- the confidence level is low; the weight for the error determined based on the decision output is lower than the weight for the error determined based on the constant modulus ( 311 ).
- a modulus square unit ( 413 ) computes the modulus square of the output of the adjustable filter ( 401 ); and a modulus square unit ( 407 ) computes the modulus square of the output of the decision engine ( 403 ).
- a subtractor ( 405 ) computes the difference between the constant ( 411 ) and the modulus square of the output of the adjustable filter ( 401 ), to provide an error signal of a CMA type.
- a subtractor ( 419 ) computes the difference between the modulus square of the output of the adjustable filter ( 401 ) and the modulus square of the output of the decision engine ( 403 ), to provide an error signal of a DMA type.
- the error signals generated from the subtractors ( 405 ) and ( 419 ) are weighted by scalers ( 421 and 423 ) and summed by the adder ( 425 ) to generate a combined error signal to drive the adaptation engine ( 409 ), which adjusts the filter ( 401 ) to reduce equalization error.
- a subtractor ( 417 ) determines the difference between the input and output of the decision engine ( 403 ). The difference is used to determine a confidence level ( 415 ) in the output of the decision engine ( 403 ). When the confidence level ( 415 ) increases, the weight for the DMA type of error increases. The confidence level is subtracted from a constant ( 429 ) by a subtractor ( 427 ) to generate the weight for the CMA type of error. Thus, the confidence level ( 415 ) controls the scalers ( 421 and 423 ) to combine the CMA and DMA types of errors.
- FIG. 5 shows example weight functions according to an embodiment of the invention.
- the weight functions ( 501 and 503 ) are computed based on the difference (d) between the input and output of the decision engine.
- ) may be a non-linear function or a linear function.
- the difference (d) indicates a level of confidence in the output of the decision engine.
- the weight function ( 503 ) for the error signal of a DMA type increases, while the weight function ( 501 ) for the error signal of a CMA type decreases.
- the weight function ( 501 ) for the CMA error signal approaches zero.
- the weight function ( 503 ) for the DMA error reaches zero.
- T 2 e.g., 1.0
- FIG. 6 shows a flow diagram of a process in a blind equalizer according to one embodiment of the invention.
- a Quadrature Amplitude Modulation (QAM) signal is received ( 601 ) to determine ( 603 ) a symbol corresponding to the received QAM signal.
- a first error in equalization is computed ( 605 ) based on a constant modulus and a second error in equalization based on the determined symbol.
- a filter is adjusted ( 607 ) to reduce error in equalization according to the first error and the second error.
- the process can be performed in an iterative way.
- the filter can be adjusted to generate the subsequent QAM that is received (e.g., in the decision engine) and used to determine the subsequent symbol.
- the filter when the difference between the received QAM signal and the determined symbol is small, the filter is adjusted according to the second, symbol-based error more than the first, constant modulus-based error.
- the filter is adjusted according to the first, constant modulus-based error more than the second, symbol-based error.
Abstract
Description
[|y|p−K]q
[|y|p−Kd]q
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Priority Applications (4)
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US11/279,200 US7599449B2 (en) | 2006-04-10 | 2006-04-10 | Hybrid modulus blind equalization for quadrature amplitude modulation (QAM) receivers |
CNA2006100875441A CN101056290A (en) | 2006-04-10 | 2006-06-14 | Hybrid modulus blind equalization for quadrature amplitude modulation (QAM) receivers |
PCT/US2006/030995 WO2007133237A1 (en) | 2006-04-10 | 2006-08-08 | Hybrid modulus blind equalization for quadrature amplitude modulation (qam) receivers |
TW095140223A TW200740157A (en) | 2006-04-10 | 2006-10-31 | Hybrid modulus blind equalization for quadrature amplitude modulation (QAM) receivers |
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US11/279,200 US7599449B2 (en) | 2006-04-10 | 2006-04-10 | Hybrid modulus blind equalization for quadrature amplitude modulation (QAM) receivers |
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CN101383792B (en) * | 2008-09-28 | 2011-04-06 | 深圳市统先科技股份有限公司 | Blind equalizing method in satellite demodulator |
CN101710884B (en) * | 2009-12-04 | 2013-07-10 | 深圳国微技术有限公司 | Method for identifying QAM mode based on channel estimation and equalization |
CN103684600B (en) * | 2012-09-14 | 2016-08-31 | 富士通株式会社 | The updating device of equalizer coefficients and method and receiver and optical communication system |
CN108199992B (en) * | 2017-12-28 | 2020-12-29 | 西安电子科技大学 | Blind equalization system and method suitable for 4096-QAM in microwave communication |
CN110581816A (en) * | 2018-06-07 | 2019-12-17 | 西南科技大学 | CMA blind equalization variable step length optimization method of MPSK signal |
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- 2006-06-14 CN CNA2006100875441A patent/CN101056290A/en active Pending
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CN101056290A (en) | 2007-10-17 |
WO2007133237A1 (en) | 2007-11-22 |
US20070237250A1 (en) | 2007-10-11 |
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